sysdb: sysdb_certmap_add() handle domains more flexible
sysdb_ldb_msg_attr_to_certmap_info() creates an empty list if there are
no domains defined, sysdb_certmap_add() should be able to handle both a
missing or an empty domains list.
Related to https://pagure.io/SSSD/sssd/issue/3500
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>